linux-kernel - Re: [PATCH 16/36] tty/vt: consolemap: check put_user() in con_get

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Message-ID: <9780cd63-5cf3-7ee0-4866-160b9de0a3e8@kernel.org>
Date:   Wed, 8 Jun 2022 10:11:12 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/36] tty/vt: consolemap: check put_user() in
 con_get_unimap()

On 08. 06. 22, 10:02, David Laight wrote:
> From: Jiri Slaby
>> Sent: 07 June 2022 11:49
>>
>> Only the return value of copy_to_user() is checked in con_get_unimap().
>> Do the same for put_user() of the count too.
>>
>> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
>> ---
>>   drivers/tty/vt/consolemap.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/vt/consolemap.c b/drivers/tty/vt/consolemap.c
>> index 831450f2bfd1..92b5dddb00d9 100644
>> --- a/drivers/tty/vt/consolemap.c
>> +++ b/drivers/tty/vt/consolemap.c
>> @@ -813,7 +813,8 @@ int con_get_unimap(struct vc_data *vc, ushort ct, ushort __user *uct,
>>   	console_unlock();
>>   	if (copy_to_user(list, unilist, min(ect, ct) * sizeof(*unilist)))
>>   		ret = -EFAULT;
>> -	put_user(ect, uct);
>> +	if (put_user(ect, uct))
>> +		ret = -EFAULT;
>>   	kvfree(unilist);
>>   	return ret ? ret : (ect <= ct) ? 0 : -ENOMEM;
>>   }
> 
> How is the user expected to check the result of this code?
> 
> AFAICT -ENOMEM is returned if either kmalloc() fails or
> the user buffer is too short?
> Looks pretty hard to detect which.

Agreed. The code is far from perfect. We might try to return ENOSPC and 
watch what breaks. (And decouple the double "?:" operator as it makes 
things only worse.)

> I've not looked at the effect of all the patches, but setting
> 'ret = -ENOMEM' and breaking the loop when the array is too
> small would simplify things.

Note that the patches try NOT to change the behavior in any way. If they 
do, it's likely a bug. They are first front cleanup. Definitely more to 
come. Either from me, or others -- patches welcome ;).

thanks,
-- 
js
suse labs